The present invention relates to optical communications networks, and more particularly to optical switching, routing, multiplexing and de-multiplexing devices.
The use of optical fiber for long-distance transmission of voice and/or data is now common. As the demand for data carrying capacity continues to increase, there is a continuing need to utilize the bandwidth of existing fiber-optic cable more efficiently. An established method for increasing the carry capacity of existing fiber cable is Wavelength Division Multiplexing (WDM) in which multiple information channels are independently transmitted over the same fiber using multiple wavelengths of light. In this practice, each light-wave-propagated information channel corresponds to light within a specific wavelength range or xe2x80x9cband.xe2x80x9d
In this specification, these individual information-carrying lights are referred to as either xe2x80x9csignalsxe2x80x9d or xe2x80x9cchannels.xe2x80x9d The totality of multiple combined signals in a wavelength-division multiplexed optical fiber, optical line or optical system, wherein each signal is of a different wavelength range, is herein referred to as a xe2x80x9ccomposite optical signal.xe2x80x9d
Because of the increased network traffic resulting from the use of the WDM technique, there is an increasing need for sophisticated optical switching and routing devices which can quickly route or re-route numerous channels amongst various optical communications lines. FIG. 1 illustrates a known apparatus that performs this function. This apparatus 300 has two control states and serves to separate channels of the wavelength spectrum applied to an input port 11 and determines which of two output ports 13, 14 are coupled to each of the channels. The input WDM signal enters the first birefringent element 30 that spatially separates horizontal and vertically polarized components of the WDM signal. The first birefringent element 30 allows the vertically polarized portion of the optical signal to pass through without changing course. In contrast, horizontally polarized waves are redirected at an angle because of the birefringent walk-off effect. The horizontally polarized component travels along a path 301 as an extraordinary signal in the first birefringent element 30 while the vertically polarized component 302 travels as an ordinary signal and passes through without spatial reorientation.
Both the horizontally and vertically polarized components 301 and 302 are coupled to a switchable polarization rotator 40 under control of a control bit. The polarization rotator 40 consists of two sub-element rotators that form a complementary state, i.e. when one turns on the other turns off, such that, in general, the rotator 40 rotates the signals by either 0xc2x0 (i.e., no rotation) or 90xc2x0. FIG. 1 illustrates one control state in which the signal 302 is rotated by 90xc2x0 so that both signals 303, 304 exiting the rotator 40 have a horizontal polarization.
The stacked waveplates element 61 is a stacked plurality of birefringent waveplates at selected orientations that generate two eigen states. The first eigen state carries a first sub-spectrum with the same polarization as the input, and the second eigen state carries a complementary sub-spectrum at the orthogonal polarization. With horizontal polarizations 303, 304 input to the stacked waveplates element 61 as shown in FIG. 1, orthogonal vertical and horizontal polarizations are generated with the first spectral band residing in horizontal polarization and the second spectral band residing in vertical polarization. With vertical polarizations 303, 304 input to the stacked waveplates element 61 (not shown) orthogonal vertical and horizontal polarizations are generated with the first spectral band residing in vertical polarization and the second spectral band residing in horizontal polarization.
The pairs of optical responses 305, 306 output by the stacked waveplates element 61 are coupled to a second birefringent element 50. This birefringent element 50 has a similar construction to the first birefringent element 30 and spatially separates the horizontally and vertically polarized components of the input optical signals 305 and 306. As shown in FIG. 1, the optical signals 305, 306 are broken into vertically polarized components 307, 308 containing the second spectral band and horizontally polarized components 309, 310 containing the first spectral band. Due to the birefringent walk-off effect, the two orthogonal polarizations that carry first spectral band 309, 310 in horizontal polarization and second spectral band 307, 308 in vertical polarization are separated by the second birefringent element 50.
Following the second birefringent element 50, the optical elements on the input side of the second birefringent element 50 can be repeated in opposite order, as illustrated in FIG. 1. The second stacked waveplates element 62 has substantially the same composition as the first stacked waveplates element 61. The horizontally polarized beams 309, 310 input to the second stacked waveplates element 62, are further purified and maintain their polarization when they exit the second stacked waveplates element 62. On the other hand, the vertically polarized beams 307, 308 experience a 90xc2x0 polarization rotation and are also purified when they exit the second stacked waveplates element 62. The 90xc2x0 polarization rotation is due to the fact that the vertically polarized beams 307, 308 carry the second spectral band and therefore are in the complementary state of element 62. At the output of the stacked waveplates element 62, all four beams 311, 312 and 313, 314 have horizontal polarization.
To recombine the spectra of the two sets of beams 311, 312 and 313, 314, a second polarization rotator 41 and a second birefringent element 70 are used. The second rotator 41 has two sub-elements that intercept the four parallel beams 311-314. The two sub-elements of the second rotator 41 are set at a complementary state to the first rotator 40. In the state illustrated in FIG. 1, the polarization of beams 311 and 313 is rotated by 90xc2x0, and beams 312 and 314 are passed without change of polarization. This results in an orthogonal polarization pair 315, 316 and 317, 318 for each spectral band at the output of the second rotator 41. Finally, a second birefringent element 70 re-combines the two orthogonal polarizations 315, 316 and 317, 318 using the walk-off effect to produce two spectra that exit at ports 14 and 13, respectively. In the operational state shown in FIG. 1, the first and second spectral bands exit at ports 13 and 14, respectively. In the other operational state of the apparatus 300, the outputs of the two spectral bands are reversed.
Although the known apparatus 300 (FIG. 1) appears to be capable of performing its intended function, the structure of the apparatus 300 entails undesirable complexity since the two different multi-segment polarization rotators 40-41 working in tandem with one another are required perform the function of switching the operational state of the apparatus 300. Further, since all the various functions of the apparatus 300 are performed by transmissive optical elements, the input port 11 and output ports 13-14 must necessarily be disposed at opposite sides of the apparatus 300. Such a disposition causes the apparatus 300 to be excessively large and creates difficulty for coupling the apparatus 300 to fiber ferrules or ribbon cables in which all the fibers are disposed within a single bundle or group. Still further, the light paths through the apparatus 300 are completely reversible, right-to-left or left-to-right. Although this functioning is acceptable for many applications, it can allow an undesired situation inadvertently reflected backward propagating light exiting the apparatus 300 through the input port 11. Thus, the apparatus 300 does not provide an optical isolation function.
Accordingly, there exists a need for an improved switchable interleaved channel separator device. The improved device should perform the functions of switching by a single polarization rotator element to reduce size and complexity of the device. The wavelength sorting function of the device should be performed by a reflection element so as to facilitate external optical coupling to the device predominantly at a single side or at adjacent sides of the device. The device should provide an optical isolation function, wherein light entering the device from either of the output ports is prevented from exiting the device through either of the input ports. The present invention addresses such a need.
The present invention provides an improved switchable interleaved channel separator device. The switchable interleaved channel separator device utilizes a reflective interferometer and one single-segment switchable polarization rotator. The reflective interferometer causes signal light paths to be reflected back upon one another so as to realize an overall reduction in size. This path reflection also enables the capability of providing optical couplings predominantly or wholly at a single side or at adjacent sides of the device.